Continental Crust formation in the Archean vs modern times

When subduction initiated and contributed to formation of Continental crust is uncertain. A crucial difference between subduction zones magma and e.g. plume related magmatism is the role of H2O in the magma formed. Subduction zones magma are frequently wet and follow a liquid line of descent (LLD) that differs from dry plume related magmas. We developed a qualitative hygrometer based on major elements that allow to distinguish between LLD formed at water saturated condition from those that formed at dry conditions. While arc magmas can by dry at times, plume related magmas are generally dry. So wet LLD are a hall mark of subduction. In this talk we will compare the modern arc record with the Archean rock record to investigate if Archean rocks formed due to a wet or dry LLD.